The invention relates to reducing dark current noise during image capture operations in a digital imaging system.
Referring to FIG. 1, a common digital image processing system includes a camera 100, a computer 102 coupled to the camera 100 by a communication link 104, and a display unit 106. The communication link 104 may be a serial bus such as the universal serial bus (USB). The display unit 106 may be any convenient display device such as a cathode ray tube (CRT) or liquid crystal display (LCD).
The camera 100 may use the imager 108 to generate an electrical representation of an optical image 110. To accomplish this, the imager 108 may include a sensor having an array of photon sensing elements. During an integration time or interval, each sensor element accumulates light energy from that portion of optical image 110 that is focused on it by camera 100 optics (not shown in FIG. 1). At the expiration of the integration interval, sensor elements indicate the intensity of the received light energy by, for example, an analog voltage signal. Camera 100 typically processes the indications from sensor elements to form a frame of digital data which may then be stored in memory internal to the camera 100 (not shown in FIG. 1), and/or transferred to the computer 102.
Typically, the frame of data does not indicate an exact duplicate of the optical image 110 due to imperfections introduced by camera 100. The camera 100 may introduce optical distortion and noise such as dark current noise. Dark current noise may be defined as sensor element current that is present even when the sensor element is not illuminated. Dark current noise tends to reduce the captured image""s dynamic range and signal to noise ratio (SNR) and places a limit on sensor element integration time. Dark current noise also tends to increase as the temperature increases, rendering many cameras substantially unusable in high temperature environments.
Some cameras attempt to cancel out dark current noise by subtracting a predetermined noise level from the intensity that is indicated by the sensor element. The predetermined noise level may be determined from, for example, an extra sensor element that is not exposed to light during image capture operations. Similarly, some cameras attempt to compensate for offset noise by subtracting a predetermined value from an analog to digital converter output.
With conventional dark current compensation schemes, a full frame exposure in the dark condition is normally used to provide for dark current compensation. This increases the capture time thereby resulting in slower back-to-back captures. Allowing for dark frame capture may also result in an additional shutter opening and closing cycle. Extra opening and closing cycles increase battery power consumption in some situations. In addition, multiple shutter opening and closing sounds per capture may cause operator confusion.
Thus, there is a continuing need for better ways for reducing dark current noise.
In accordance with one aspect, a method for reducing dark current noise in an image sensor includes measuring the forward voltage of a diode on the integrated circuit including the image sensor. An image is captured using the sensor. The forward voltage measurement is used to correct the image sensor for dark current noise.